1. Field of the Invention
This invention relates to a method and apparatus for preventing the corrosion of underground metallic gas carrier pipes, such as natural gas pipelines. More particularly, this invention relates to a method and apparatus for preventing the corrosion of underground metallic gas carrier pipes using cathodic protection. More particularly yet, this invention relates to a method and apparatus for preventing the corrosion of underground metallic gas carrier pipes disposed within an outer casing pipe using cathodic protection.
2. Description of Related Art
The corrosion of metals is the result of electrochemical reactions, that is, chemical reactions accompanied by a flow of electrical current. The electrochemical reactions occur through a combination of chemical reactions in the exchange of electrical charges (current) between areas where these chemical reactions are occurring, in effect, an electrochemical cell. Cathodic protection is a method for preventing the corrosion of metals which utilizes a flow of direct current electricity to interfere with the activity of the electrochemical reactions responsible for the corrosion. Corrosion is prevented by coupling a metal with a more active metal when both are immersed in an electrolyte and connected with an external path. In this case, the metal being protected becomes a cathode electrode.
Every metal disposed in an electrolyte develops an electrochemical potential due to the free energy of the atoms in the metal. To prevent anodic reactions from occurring due to electrochemical reactions on the metal, electrons must be prevented from leaving the metal. Because electrons can only flow from an area of high, negative, potential to an area with lower, negative, potential, connection of the metal to be protected to a source of the more negative electrons effectively prevents the anodic reactions on the metal to be protected, thereby preventing corrosion. In this case, the flow of electrons is from the external source to the metal being protected.
Cased metallic pipe with an open, air filled, annular space cannot be cathodically protected. Since 2002, integrity assessments of “covered segments” of gas transmission lines through the use of in-line inspection, pressure testing, or direct assessment, have been required. When a transmission rated gas line is in a high consequence area, it is required that a baseline assessment be carried out by 2012 followed by an assessment every seven years or sooner.
Most cased pipe sections owned by local distribution companies that fall under the regulations requiring integrity assessments cannot be in-line inspected or pressure tested. Although they can be assessed by direct assessment, the casing around the pipe “shields” the gas carrier pipe from the use of nearly all inspection tools, such as direct current voltage gradient, alternating current voltage gradient, and electromagnetic pipe inspection, e.g. pipeline current mapper. In addition, the most important survey, that is, highest weighted, for direct assessment is the close interval survey which measures the cathodic protection levels on the pipe relative to its surrounding environment. However, the close interval survey cannot be applied to conventional cased pipe sections because they do not have cathodic protection applied.